Magnetic sensors or pickups, by converting mechanical motion into electric energy, may be used to measure speed or as a source of electric signals. A typical magnetic pickup for measuring rotational speed is shown in FIG. 1 of U.S. Pat. No. 3,794,855. An elongated permanent magnet is contacted at one pole to one end of an elongated ferromagnetic pole piece which carries an electric coil. As the teeth of a ferrous rotor move past the other end of the pole piece, the overall reluctance of the magnetic circuit changes, thus inducing an AC voltage in the coil. Sometimes the magnetic pickup is mounted within a ferromagnetic casing which provides an easy return path for the magnetic flux from the pole piece to the magnet, as in U.S. Pat. No. 3,980,913. Usually the permanent magnet is of the Alnico type. Other permanent magnet materials have been little used in magnetic pickups of the U.S. Pat. No. 3,794,855 design, either being too weak magnetically or too expensive to be commerically competitive.
Numerous other designs have been suggested for magnetic pickups. That of U.S. Pat. No. 4,045,738 is similar to that of U.S. Pat. No. 3,794,855 except that the magnet and pole piece are reversed, and the magnet is a very thin disk since it is a rare earth based permanent magnet. In U.S. Pat. No. 3,492,518, a coil wound on a ferromagnetic pole piece is positioned within a permanent magnet in the shape of a sleeve. In U.S. Pat. No. 4,061,938, a pair of toothed ferrous rotors and a permanent magnet are mounted on a shaft. As the teeth move past the elongated pole pieces of a pair of T-shaped yokes, pulses are generated in a magneto-electric transducer positioned between the yokes.
U.S. Pat. No. 3,878,432 illustrates two additional designs. In one, a coil is wound on a cylindrical permanent magnet which is mounted within a cup-shaped housing formed by an iron cylinder and an iron disk which together serve as pole pieces and a return path. In the other, a pair of pole pieces are affixed to the circular surfaces of a cylindrical permanent magnet on which a coil is wound.
While most of the aforementioned magnetic pickups are primarily based on rotary motion, a magnetic pickup for sensing linear motion is shown in U.S. Pat. No. 3,945,459. One species employs a ring-shaped permanent magnet having one pole at its inner-facing surface and the other pole at its outer-facing surface. Reluctance changes are sensed by a coil positioned coaxially with the magnet within a ferrous cylinder when a notched or toothed shaft moves coaxially within the coil and magnet. The patent apparently does not identify the permanent magnet which, unlike the permanent magnets of the pickups mentioned above, is magnetized in the radial directions. However, U.S. Pat. No. 3,127,461 teaches a ring-shaped permanent magnet comprising matrix-bonded permanent magnet particles of barium, strontium and/or lead ferrite which have been aligned to have preferred directions of magnetization perpendicular to its axis. It is magnetized to have one pole at its inner-facing surface and the other pole at its outer-facing surface.